Audio peak limiting has been a fundamental part of the production, transmission and reproduction of audio for over 50 years. For example, if multiple digital audio signals are mixed in the digital domain without scaling, during playback it is possible for the sum of the audio data to exceed the maximum digital signal that can be represented during the digital-to-analog conversion. The result is a clipped output analog waveform, which audio peak limiting aims to avoid.
Audio peak limiting is discussed in U.S. Pat. No. 8,488,811 B2, the “'811 patent”, the entire contents of which are hereby incorporated by reference. The '811 patent discloses a two-stage audio peak limiter which includes a slow-gain reducer and a fast-gain reducer. The slow-gain reducer receives an audio signal, calculates a slow gain and multiplies the audio signal by the slow gain to produce an intermediate audio signal. The fast-gain reducer receives and buffers the intermediate audio signal, performs peak detection on the buffered intermediate audio signal, uses the output of the peak detection to calculate a fast gain, and multiplies the buffered intermediate audio signal the by the fast gain to produce an output audio signal. Broadly speaking, the slow gain can change only slowly in order to avoid sudden changes in loudness in the final rendering of the audio signal, and the fast gain acts as a kind of backup or failsafe to quickly change the total gain applied to the audio signal when necessary to avoid clipping as a result of the slow gain not changing quickly enough to deal with a high signal peak.
The length of the buffer in the audio peak limiter is an important design parameter. Too long, and the latency introduced by the audio peak limiter becomes too high. Too short, and the gain applied by the audio peak limiter will not be smoothed to a sufficient extent, particularly if the audio signal includes significant low-frequency components (e.g., sustained bass).
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